The present invention relates to an anticorrosion coating composition which can be used, in particular, to protect threaded devices. The invention also relates to a process for its application as well as to threaded devices coated in this manner.
There are known processes for coating metal substrates with compositions containing hexavalent chromium, which is frequently supplied by chromic acid, in order to impart corrosion resistance to such as metal nut-and-bolt substrates.
French Pat. No. 2,008,770 (U.S. Pat. No. 3,671,331) describes an improvement to this type of composition in which a powdered metal is added in order to improve corrosion resistance. French Pat. No. 2,149,434 (U.S. Pat. No. 3,907,608) describes another improvement to the above compositions making it possible to improve the characteristics of the coating produced and to facilitate its application. U.S. Pat. No. 4,266,975 describes improved coating compositions containing substances such as boric acid, and as also described in French Pat. No. 2,008,770.
In general, the application of an anticorrosion coating to bolts, screws or nuts, whether this coating be of the electrolytic coating type, a paint coating or a coating produced from solutions of hexavalent chromium, modifies the characteristics of the threaded zone. These modifications are, in general, excessive thicknesses, modifications of surface properties (slip, adhesion, reliability of the coating) or thread damage.
These modifications result in inadequate tensions in the threaded portions to produce a reliable assembly, even with high clamping torques.
Very frequently, the problem is made still worse if the same parts are once again loosened and retightened; the tensions obtained are then still lower and consequently the assemblies are still less resistant to external stresses.
Thus, in the field of threaded components, where the standards are increasingly exacting, especially on automated assembly lines, it would be desirable to have available compositions which, in addition to outstanding corrosion resistance, could impart to the threaded elements properties making it possible to obtain high tensions in the assembly.
Thus, the coating should, preferably, make it possible to obtain a specified tension value which is generally equal to 80% of the elastic limit of the bolt.
In addition, it is advantageous to be able to obtain good reliability of the values of the tension/clamping torque relationship from one clamping operation to another with the same bolt or different bolts.
While the coatings according to the prior art are perfectly satisfactory from the point of view of anticorrosion properties, they have occasionally been found inadequate from the point of view of the mechanical properties outlined earlier.